An electronic control unit (ECU) used in a vehicle is powered by a voltage supplied from a power supply circuit. When a switch such as an ignition switch, a key switch, or a main power switch of the vehicle is in an ON position, the ECU operates in a normal operation mode. Therefore, the power supply circuit needs to output an enough current (e.g., hundreds of milliamperes) to allow the ECU to operate in the normal operation mode. In contrast, when the ignition switch is in an OFF position, the ECU operates in a low power consumption mode. Therefore, the output current from the power supply circuit is very small.
The power supply circuit includes a high accuracy power supply circuit used in the normal operation mode and a low accuracy power supply circuit used in the low power consumption mode. Although the high accuracy power supply circuit requires a large current, the high accuracy power supply circuit generates an accurately regulated voltage by using, for example, a bandgap reference circuit. In contrast, although the low accuracy power supply circuit requires a small current, the low accuracy power supply circuit generates a poorly regulated voltage by using, for example, a zener diode. Thus, a current (i.e., dark current) consumed when the ignition switch is in the OFF position is reduced.
U.S. Pat. No. 6,400,589 corresponding to JP-A-2001-268787 discloses a power supply circuit having a main DC-DC converter and a sub DC-DC converter. When the ignition switch is in the OFF position, the main DC-DC converter is turned off and only the sub DC-DC converter operates to supply a dark current to a low voltage load. When the ignition switch is in the ON position, the main DC-DC converter is turned on and supplies a required power and the sub DC-DC converter supplies the dark current.
FIG. 7 shows a circuit diagram of a conventional power supply integrated circuit (IC) 1. The conventional power supply IC 1 includes a low-accuracy power supply circuit 2 and a high-accuracy power supply circuit 3.
The low-accuracy power supply circuit 2 is powered by a battery voltage VBAT and includes a voltage generation circuit 5 and an emitter follower circuit 6. The voltage generation circuit 5 has a constant current circuit 4, a diode D1, and a zener diode D2, which are connected in series. The emitter follower circuit 6 has transistors Q1, Q2.
The high-accuracy power supply circuit 3 is powered by a battery voltage VB supplied through an ignition switch and includes a bandgap reference circuit 7 and a voltage output circuit 8. The voltage output circuit 8 has an operational amplifier 9, a feedback circuit 10, and a transistor Q3.
In the conventional power supply IC 1, when the ignition switch is in the OFF position, only the low-accuracy power supply circuit 2 operates. Therefore, a voltage Vo output from the conventional power supply IC 1 is given by the following equation:Vo=Vz−VF
In the equation, Vz represents a zener voltage of the zener diode D2 and VF represents a forward voltage of a PN junction.
When the ignition switch is in the ON position, each of the low-accuracy power supply circuit 2 and the high-accuracy power supply circuit 3 operates. The high-accuracy power supply circuit 3 performs a feedback control that maintains the output voltage Vo at a constant level, for example, 5 volts (V). Therefore, when the zener voltage Vz minus the forward voltage VF is less than 5 V (i.e., Vz−VF<5 V), the high-accuracy power supply circuit 3 mainly works to maintain the output voltage Vo at 5 V. In contrast, when the zener voltage Vz minus the forward voltage VF is greater than 5 V (i.e., Vz−VF>5 V) and a current output from a terminal 1a of the conventional power supply IC 1 is small, the low-accuracy power supply circuit 2 mainly works. Consequently, the output voltage Vo is the zener voltage Vz minus the forward voltage VF (i.e., Vo=Vz−VF>5 V). Because the low-accuracy power supply circuit 2 has a low capability to output the current, the output voltage Vo decreases with an increase in the output current due to an increase in a load. Therefore, the output voltage Vo decreases to 5 V and is maintained at 5 V by the high-accuracy power supply circuit 3.
As described above, when the zener voltage Vz minus the forward voltage VF is greater than 5 V, the conventional power supply IC 1 falls into an unstable condition where the output voltage Vo varies with the output current.
Therefore, the conventional power supply IC 1 is configured such that the zener voltage Vz minus the forward voltage VF is lower than 5 V, i.e., the output voltage of the low-accuracy power supply circuit 2 is lower than that of the high-accuracy power supply circuit 3. Further, when manufacturing and temperature variations in the zener voltage Vz are considered, the output voltage Vo that is output when the ignition switch is in the OFF position needs to be set lower than 5 V.